docs: def fetch_entities(self):
          """Fetch entities for which we have data."""
          query = text(
              """
              SELECT entity_id
              FROM states
              GROUP BY entity_id
              """
          )
          response = self.perform_query(query)
  
          # Parse the domains from the entities.
          entities = {}
          domains = set()
  
          for [entity] in response:
              domain = entity.split(".")[0]
              domains.add(domain)
              entities.setdefault(domain, []).append(entity)
  
          self._domains = list(domains)
          self._entities = entities
          print("There are {} entities with data".format(len(entities)))